Based on the weak absorption measurement system with surface thermal lens (STL) technique, different power pump laser is used to irradiate the sample and record the intensity distribution of probe beam with beam quality analyzer. Making the pixel coordinates of the strongest intensity point of the probe beam as the center, by taking multiple pixel from both sides of the center, different sizes of photoelectric signal receiver can be represented and photo- thermal signal intensity which obtained by different sizes of photoelectric signal receiver can be analyzed. The photo-thermal signal as a function of size of photoelectric signal receiver is experimental measured. The experimental results show that with the size of photoelectric signal receiver increasing, the coverage area of detection exceeds the probe beam peak area，and then the photo-thermal signal deviates from linearity gradually, which enlarges the measurement error. Therefore, to improve the accuracy of measurements，the actual size of photoelectric signal receiver should be minimized on the premise of guaranteeing measurement limit.